1. Field of the Invention
The present invention relates to an oxygen/inert gas generator capable of use in aircraft and more particularly, the present invention relates to a hybrid On Board Oxygen Generating System (OBOGS) and an On Board Inert Gas Generating System (OBIGGS).
2. Description of the Related Art
The present invention addresses the need aboard many aircraft for oxygen-enriched and nitrogen enriched gases. Oxygen is needed for passengers in the event of decompression of the aircraft passenger compartment to pressures equivalent to altitudes greater than 10,000 feet. Some passengers with lung disorders may also require oxygen. Nitrogen is needed to inert the space above the fuel in the fuel tanks, also called the ullage, to reduce the possibility of explosive conditions. Nitrogen can also be used to help extinguish fires in the cargo bays.
Two technologies have been commonly used to provide oxygen-enriched and nitrogen-enriched gas aboard aircraft from pressurized air that is available from the engine bleed air sources. An On Board Oxygen Generating System (OBOGS) and an On Board Inert Gas Generating System (OBIGGS) employing molecular sieves and a pressure swing adsorption process (PSA) have been widely used as one technology to respectively provide oxygen-enriched and nitrogen-enriched gases. A second technology used to provide nitrogen-enriched gas is commonly referred to as Hollow Fiber Membrane (HFM) or permeable Membrane (PM) technology.
The approach using molecular sieves and the PSA process is generally accepted as the best way to generate oxygen-enriched gas aboard aircraft when the purity requirements are less than about 95% oxygen. The PSA and HFM technologies used to produce nitrogen-enriched gas each have particular advantages and disadvantages depending upon the available aircraft resources, gas flow rates and purity of nitrogen-enriched gas desired, and the environmental conditions such as temperature, air supply pressure and surrounding absolute pressure, or altitude. Each parameter affects the performance of each technology differently. For instance, PSA-based technology generally performs better at temperatures of about 70 degrees Fahrenheit, while the HFM technology performs better at temperatures of about 160 degrees Fahrenheit. There are many parameters that must be considered to best utilize these technologies most effectively for each application.
Many attempts have been made to find synergy in applying the gas separation technologies aboard aircraft to generate oxygen-enriched and nitrogen-enriched gases that use less engine bleed air and electrical power and/or reduce the size and weight of the gas separation systems. One of the successful attempts is the combination OBOGS/OBIGGS used on the V-22 aircraft. This system uses two types of molecular sieves and the same PSA process to generate oxygen-enriched gas for aircrew breathing and nitrogen-enriched gas to inert the fuel tank ullage. This two-gas system provides synergistic benefits in reduced size and weight by sharing common system components such as the inlet filter, pressure reducer, PSA cycling valve, and control electronics to produce both gases.
The oxygen concentrating PSA technology and the nitrogen concentrating HFM technology each have limits to the purity of the gases produced and the concentration of the waste gases that are enriched with other primary constituents in air that is not desired as the enriched product gas. For example, the PSA-based OBOGS separates oxygen-enriched gas for breathing, but also must exhaust nitrogen-enriched gas that is commonly discarded to the surrounding atmosphere. Likewise, the HFM-based OBIGGS separates nitrogen-enriched gas for inerting purposes, but also must exhaust oxygen-enriched waste gas that is commonly discarded to the surrounding atmosphere.
Many have thought that it would be desirable to use the nitrogen-enriched exhaust gas from the OBOGS as inlet gas to the HFM OBIGGS to enhance its performance, or the oxygen-enriched exhaust gas from the OBIGGS as inlet gas to the PSA OBOGS to enhance its performance. However, the PSA and HFM technologies each require pressure differentials from their inlets to their exhaust ports for best performance. Generally, the PSA-based OBOGS and OBIGGS processes work well with 20 to 60 psig of inlet air pressure with the exhaust free to vent to the surrounding atmosphere. The HFM OBIGOS technology works well with inlet air pressure of about 25 psig to about 100 psig, with the higher pressures preferred.
The inlet air pressure available on board aircraft typically ranges from about 20 psig to 75 psig, with most aircraft air supplies in the lower half of that range. Therefore, a typical aircraft air supply of 30 psig works fairly well as a supply for each OBOGS and OBIGGS technology, but coupling the exhaust from the OBOGS to the inlet of the OBIGGS would degrade the OBOGS performance. This would limit the free exhausting of the waste gas during the PSA process, while also reducing the inlet pressure to the HFM OBIGGS. Each may have only about 15 psig of energy available to drive each process. This is well below the preferred pressures desired for each process for near-optimum performance.
A compressor could be added between the OBOGS exhaust and the OBIGGS inlet to increase the effective pressure differentials available for each gas-separation process. U.S. Pat. No. 4,681,602 discloses such an integrated system for generating inert gas and breathing gas on an aircraft that utilizes a compressor to compress an output from one gas generator prior to inputting it to the other gas generator. However, the compressor's size, weight and cost penalties more than offset the gas-separation performance gains.
U.S. Pat. No. 6,319,305 discloses a gas generating system for generating a supply of oxygen and a residual gas including a first gas separation device for separating oxygen-enriched gas from a supply gas and leaving a residual gas. The first oxygen-enriched gas from the first gas separation device is inputted to a second gas separation device for further separating oxygen gas from the first oxygen-enriched gas. The second gas separation device generates a product gas that is at least highly oxygen-enriched and a further residual gas, with at least one of the first and second gas separating devices including a ceramic membrane through which gas ions diffuse.